The instant invention relates to fiber optic transmitters, and more particularly, the instant invention relates to an integrated fiber optic transmitter which creates and amplifies a modulated fiber optic signal.
Fiber optic transmitters and amplifiers are well known in the art. In a typical fiber optic communication system, a transmitter and an amplifier are coupled together to produce and amplify a signal to be transmitted over a long distance. Referring to FIG. 1, a semiconductor laser transmitter 10 is utilized to create a transmission signal. The transmitter includes a semiconductor laser 12 having a specified light wave output, i.e. 1550 nm, an optical isolator 14, a modulator 16 to modulate the signal output, and an output connector 18. The output of the transmitter 10 is thereafter input into a fiber optic amplifier 20 to amplify the strength of the light wave for transmission of the signal over a great length of fiber. The amplifier 20 includes an input connector, 22 a separate pump laser 24 having a pump output, i.e. 980 nm, and a wavelength divisional multiplexer (WDM) 26 to combine the signal output with the pump output. The combined signal output and pump output are passed through a rare-earth doped fiber 28 which absorbs the energy from the pump wavelength to amplify the signal output. The amplifier 20 further includes an optical isolator 30 and an output connector 32 for connecting the amplifier into a fiber trunk which will carry the signal.
In this invention, it is an object to provide an integrated approach to a fiber based optic transmitter, which utilizes a single semiconductor laser to pump a rare earth doped fiber laser to generate a single longitudinal mode, single polarization optical laser signal output, and fully use the remnant pump power to amplify the modulated optical laser signal by means of a rare earth dopes fiber, the amplifier being positioned after the modulator. The premise of the invention is to separate the remnant pump output of a fiber laser and utilize this remnant pump to later amplify the modulated signal in a single integrated unit. Also, by placing the amplifier after the modulator (which requires linearized polarized input light) allows the design to use commonly available, inexpensive, non-polarizing components for the amplifier. This design improved overall transmitter performance by eliminating the polarization extinction degradation from a polarization maintaining amplifier. It also significantly reduces the cost of the amplifier since the polarization maintaining components, e.g. isolator, rare earth doped optical fiber, WDM etc., are markedly more expensive compared to other non-polarization maintaining components.
More specifically, a fiber laser produces a signal output and a remnant pump output from a single pump source. The remnant pump output is divided out of the signal path, the signal output is modulated and then recombined with the remnant pump output. The recombined pump and modulated signal output are passed through an erbium doped fiber which absorbs the pump output to amplify the modulated signal output. More specifically, the output from the fiber laser is input into a first wavelength division multiplexer (WDM) which divides the pump output from the signal output. The first WDM outputs the signal output on a first output trunk and the pump output on a second output trunk. A modulator is connected to the first output trunk of the first WDM for modulating the signal output. A second WDM having a first input connected to the output of the modulator and a second input connected to the second output trunk (pump output) of the first WDM recombines the signal output and the pump output after modulation of the signal output. The recombined signal and pump outputs are then passed through the rare-earth doped fiber to amplify the modulated signal output.
The basic creation and amplification arrangement is easily incorporated into a variety of single and multiple wavelength transmitters which no longer require secondary pump sources to create additional signal wavelengths or provide additional pump power. For example, in a multiple wavelength transmitter, a single pump laser combined, either in parallel or series, with a plurality of fiber lasers is operative for creating multiple signal wavelengths in a single system. The signal wavelengths are separated from the remnant pump output, separately modulated, and then recombined with the remnant pump power to amplify all of the signals.
Accordingly, among the objects of the instant invention are: the provision of an improved fiber optic transmitter which reduces the number of physical connectors in a system; the provision of an improved fiber optic transmitter which integrates creation and amplification of an optical signal into a single operating unit; the provision of such a transmitter wherein only a single pump laser is utilized to create and amplify a signal; the provision of such a transmitter wherein a fiber laser is utilized to create a signal output and a remnant pump output; the provision of such a transmitter wherein the remnant pump output is divided from the signal output, and later used to amplify the same signal output after the signal output is modulated; and the provision of a multiple wavelength fiber optic transmitter wherein a single pump laser can be used to create and amplify multiple wavelengths.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.